1. Field of the Invention
The present invention relates to a semiconductor device having a sensor element and a circuit including a thin film transistor (hereinafter referred to as a TFT) and to its manufacturing method.
It is noted that the semiconductor device herein indicates a general device that can function by utilizing semiconductor characteristic. An electro-optical device, a semiconductor circuit, and an electronic apparatus are included in the semiconductor device.
2. Related Art
Conventionally, there are a sensor element using a single-crystal silicon substrate and a sensor element using an amorphous silicon film as a solid imaging element.
The characteristic of the sensor element using the single-crystal silicon substrate is that high output can be obtained by manufacturing an output amplifier circuit on the single-crystal silicon substrate and by integrating this output amplifier circuit with the sensor element. However, there are problems that the completed parts to be packaged don't have a compact shape because a wavelength sensitivity correcting filter is necessary and that the characteristic of the sensor element using the single-crystal silicon substrate varies widely.
On the other hand, the characteristic of the sensor element using the amorphous silicon film is that a correction filter such as an infrared light cut filter is not necessary because the wavelength sensitivity of this sensor element is close to that of the human eye. However, the output value of the sensor element is limited because it is not amplified. Therefore, since the output value of the sensor element is small, it is easily affected by the noise of another signal or the like. The output value of the sensor element depends on the absolute amount of the sensor element such as the area or the thickness of the sensor element. Therefore, it is necessary to enlarge the area of the sensor element using the amorphous silicon film in order to increase the output value of the sensor element using the amorphous silicon film.
It is possible to amplify the output of the sensor element using the amorphous silicon film by providing an operational amplifier externally to the sensor element using the amorphous silicon film. In this case, however, there is another problem that the number of external parts increases so that the sensor circuit becomes large.
The photosensitivity of the sensor element using the amorphous silicon film is not more than 1/10 of the sensor element using the single-crystal silicon substrate. Therefore, when the sensor element using the amorphous silicon film is employed in the display device requiring a large area such as a liquid crystal projector, the display device is easily affected by the noise because a lead wiring is long in the large display device. Therefore, since a shield covering a wiring is necessary in order to use the sensor element using the amorphous silicon film in the large display device, the cost for manufacturing the display device increases.
The present inventors have submitted patent documents 1 to 4 concerning the semiconductor device having a sensor element and a circuit including a TFT over a glass substrate.    [patent document 1] Japanese Patent Laid-Open No. H6-275808    [patent document 2] Japanese Patent Laid-Open No. 2001-320547    [patent document 3] Japanese Patent Laid-Open No. 2002-62856    [patent document 4] Japanese Patent Laid-Open No. 2002-176162